ABSTRACT
The Tala headrace tunnel comprises a 100% concrete lined, 22 km, 6.8 m internal diameter component of the 1020 MW run-of-river hydroelectric scheme located along the Wangchu River in Bhutan that was totally commissioned in early 2007. Construction of the headrace tunnel experienced some very adverse geotechnical conditions that required design modifications including a reduction of the internal diameter along two areas and a re-alignment of a 1200 m portion of the tunnel. The headrace tunnel operated without any issues until late 2018 when fragments of concrete started to enter into the powerhouse units. An unwatered inspection of the headrace tunnel was performed in early 2021 using a remote operated vehicle (ROV) that provided a survey of the entire tunnel and an indication of the structural conditions and integrity of the tunnel. Based on an independent technical evaluation of the findings of the ROV inspection and the perceived associated risks for continued operations, a decision was made to dewater the tunnel and perform a manual inspection. The findings of the manual inspection were found to be in good agreement with the interpretation of the observations from the ROV inspection that comprised damage in the form of major cracking and exposure of reinforcement at multiple areas of the concrete lining consistent with the locations of reduced internal diameter. Advances in sonar technology and ROVs have now allowed reasonably accurate identification and interpretation of structural damages in operating hydroelectric tunnels.
